Bell type furnaces are generally used in industry for the annealing of coiled strip and rod or wire coils as well as annealing other material. These furnaces provide the varied annealing cycles for various grades of steel and non-ferrous products. They are well suited to large production requirements as in the large steel mills or to small plants providing specialty annealed products. Much of this production requires more specialized annealing procedures and particularly control of the protective atmosphere for carburizing, spheroidizing, anneal with decarburization, etc.
Present standard bell furnaces utilize seals on the inner cover or retort of solid material (compressed sponge rubber or the like), or liquid seals (oil or water for non-ferrous material), and sand or ceramic wool seals. None of these seals is suitable for vacuum annealing since they have limited sealing properties, generally being effective to seal against only very low pressure difference, e.g. less than about 6" to 8" w.c. Presently certain special products require very pure atmosphere and vacuum furnaces with "O" ring clamp down seals are being utilized. These vacuum furnaces and "O" ring base to inner cover seals require precision machining and are very costly. In addition, large diameter sizes increase the cost of machining and the thickness of inner cover retorts which are stainless steel, and therefore, quite costly.
The prior art bell furnace annealing system consists of multiples of bases suitable for supporting the coil charges and the bases include large volume circulating fans to circulate the protective atmosphere during heating and cooling of the metal. Since cooling times required to cool the charge are generally much longer than the required heating time, each base load is covered by the inner cover or retort of stainless steel and sealed to the base. To prevent oxidation or decarburization of the metal, a purge of the air is made by flushing nitrogen or other oxygen free gas through the charge area until a safe protective control atmosphere is obtained. The bell furnace, which is portable like the inner cover, is then placed on the base over the inner cover containing the work coils. The furnace which provides the heat is operated and controlled by temperature and time, then removed by crane and placed on another base and inner cover which has been purged and ready for heating. The inner cover and charge which are on the base continues to cool with a flow of protective atmosphere until a temperature is reached suitable for uncovering. The atmosphere is stopped, the inner cover unsealed during removal from the base, and the coils of steel or work are removed from the base. Another charge is loaded and the procedures repeated for the new charge.
The purging of the inner cover and work particularly to remove oxygen as part of the air is critical to obtaining a suitable atmosphere prior to heating the work. Normally once the inner cover is sealed to the base, this purging of the air is accomplished by flowing large volumes of nitrogen atmosphere into the inner cover and exhausting it through an outlet pipe until oxygen is low enough to permit heating without oxidation of the work. It is also important to reduce the dewpoints of the atmosphere to the point where oxidation or carbon reduction will not take place. This requires more purging and a long time is consumed, rquiring many turnovers in the volume of gas removed until a sufficiently low level of oxygen and water vapor in nitrogen or other inert atmosphere is achieved.